Poach or grill the chicken breast. Cut into cubes.
Place the potatoes in a pan, cover with water, season with some salt. Bring to the boil and cook until tender (check by pricking a potato with a pointy knife: if there is resistance, the potatoes are not cooked; if it slides in they are cooked).
Make the sauce by combining the mayonnaise, gherkin juice and mustard. Season strongly with salt and pepper, as all the ingredients in the salad will have only a thin coating of this dressing.
Mix the chicken, potatoes, gherkins, celery, apple and sauce carefully. Because the potatoes are still a little warm, they will soak up some of the sauce and will give the salad a lukewarm serving temperature. Some people like the salad fridge-cold, but this will go at the cost of taste.